This invention is directed to the problem of low efficiency of heat pump systems due to low ambient temperature.
It is well known that a heat pump, in heating mode, will reach a "balance point" at some value of ambient air temperature. Simply put, this point is reached when the heat pump system requires supplemental heat in order to maintain the inside air temperature demanded by the thermostat. Some systems have been employed in which the heat pump is simply switched off at this "balance point" with all heat thereafter being supplied by a more conventional heating system such as a furnace. Still others have employed control systems in which the heat pump system is still utilized down to its limit of ambient temperature (e.g., 10.degree. F.) while increasingly supplementing its heat output, below the "balance point", by more conventional means such as electrical resistance heaters, etc.
Whereas such systems have also employed defrosting heaters for the outside coil (essential to avoid "blinding" of the coil and to retain good heat transfer with the circulated ambient air), it has not been recognized that the efficiency of a heat pump system may be artificially restored under low ambient air temperatures to a sufficiently high value, with minimal heat input, as to justify, economically, this sort of "bootstrapping".
Thus, in a conventional system, when the heat available for extraction from ambient air has reached such a low value as to produce relatively low efficiency for the system, heat is applied directly to the outside coil in such limited quantity as (1) artificially restores the efficiency to a much higher value and (2) does so with a net decrease in operating cost.